That sounds about right. The threaded area is quite short on every lockring I've come across.

Are you sure that you are properly aligning your top cog? If it's an 11T cog, make sure there is no grit inside of the splines that could become trapped between the freehub and cog when you install it. The 11T cogs are not open all the way through like 12T cogs are (though I don't know which cog you are using).

Have you tried installing just the top cog and seeing if the lockring will go on?